An optical instrument, such as a telescope, placed in an orbiting observation satellite, may become defocused, particularly under the thermoelastic effects and the offgassing of the structure.
The need to have a good contrast in the images implies accurately measuring this defocusing and installing a refocusing device that will be activated according to the result of the measurement.
According to the prior art, these defocusing determination problems were solved in different ways:
by comparison with simulations made with different defocusing levels, but such a method is not very accurate,
by analyzing the wave surface, but this can be implemented only with a complex system,
by making use of a measurement method consisting in varying the refocusing mechanism and selecting the best focusing, but implementing this is complex,                using a specific device or a line optical divider (called “DIVOLI”).        
This device requires peer detectors to be perfectly optically aligned on the ground and also introduces glass into the optical combination.
Moreover, a method called “phase diversity” is known, which, starting from an image of an object and the image of the same object defocused relative to the first image with a known defocusing delta using an additional sensor usually defocused by a fraction of a wavelength, or using an action on the focusing mechanism, implements a specific algorithm used to return not only to the defocusing but also to the other conventional optical aberrations.
The document JP 10 227971 A discloses a focusing device comprising an optical flux divider, in which a detection strip is moved relative to a better optical focusing plane, which complicates its implementation and makes it sensitive to accelerations, and impairs its reliability in a space environment.
Furthermore, the documents U.S. 2005/0270611 A1 and U.S. 2007/0102620 disclose microscopes with focusing determination devices comprising light beam splitters and a piezoelectric mechanism for the device of the first of these two documents, which makes these devices relatively bulky and difficult to transpose for space use with telescopes.